The environmental and photo-physiological control of microphytobenthos primary production on an intertidal mudflat
Microphytobenthos are important primary producers on tidal flats - ecosystems which are characterised by steep physical and chemical gradients. In this study some of the environmental factors that control the distribution and primary production of benthic microalgae were investigated in a series of field surveys conducted on Hythe intertidal mudflat, Southampton Water. The ecological and photo-physiological responses of microphytobenthos to changes in physical factors were studied in the field and in controlled laboratory experiments. Results from a weekly sample program at four stations along an intertidal transect were used to address the hypothesis that changes in incident irradiance defined by a combination of meteorological factors and the tidal cycle cause significant changes in the biomass of microphytobenthos. The light regime at each station was characterised by a model which combined the changes in daylight irradiance with the timing and duration of low tide. At the low shore stations total light availability during low tide was limiting in spring and autumn/winter, when daily photo-periods ranged between 0 and 6 hours. Microalgal biomass exhibited a strong seasonality with a distinct spring bloom in April which coincided with high irradiances at spring low tide. At the high shore station which was exposed to three times longer photo-periods light availability did not correlate with microalgal biomass at any time of the year. Biomass was three times higher at this station than at the three low shore stations and showed little seasonal variation. Taxonomic marker pigments were used to describe the changes in microphytobenthos composition. High correlation between chlorophyll a and fucoxanthin on a temporal and spatial (horizontal and vertical) scale combined with taxonomic analysis gave evidence that diatoms were the main primary producers on Hythe intertidal mudflat. Chlorophyll a and Ik (defined as PBmax/ aB) correlated strongly with incident irradiance at low tide, indicating that photoadaptation of benthic microalgae was occurring. A spatial comparison of photosynthetic parameters showed that benthic microalgae inhabiting the low shore compensated for reduced photoperiods by increasing their photosynthetic capacity (PBmax) and efficiency (aB) and decreasing their Ik. Photosynthetic parameters of microphytobenthos significantly changed during the low tide period under controlled laboratory conditions, suggesting that an endogenous photosynthetic rhythm was present, which caused an increase in PBmax and ocB towards the middle of the photoperiod. Irradiance levels at low tide enhanced the short-term variation in photosynthetic parameters due to photo-adaptation. The change in rates of photosynthesis was accompanied by a significant alteration of the cellular pigment composition, as the photoprotectant xanthophyll diatoxanthin increased and diadinoxanthin decreased in proportion. Short-term changes in cellular pigment composition induced in the laboratory were greater than seasonal pigment adaptation of microphytobenthos isolated from the field. Variation of PBmaxand ocB during one photoperiod occurred on a similar scale to the seasonal and spatial changes, suggesting that short-term photo-physiological adaptations are an important feature of microphytobenthos, aiding their successful survival in the intertidal environment, where strong gradients of incident irradiance occur on small temporal and spatial scales.